1. Field of the Invention
The present invention relates to a welding material for austenite stainless steel having a high Si content and the method of application therefor and, more particularly, to a welding material which is able to provide the weld metal with excellent toughness and corrosion resistance to nitric acid and the method of application therefor.
2. Description of the Prior Art
Environments containing nitric acid, particularly when the temperature and concentration are high, exhibit an oxidability which is so strong that stainless steels exhibit an over-passivity. In fact, ordinary stainless steels cannot maintain the passive film in such an environment and experience heavy intergranular corrosion.
On the other hand, silicious materials exhibit an high durability in such a strong oxidizing environment. Therefore, the use of cast iron having a high Si content and glass lining materials is convenient. These materials, however, are impossible to weld and are not suitable for the production of large-sized structures. In addition, these materials are generally fragile and lacking in impact resistance.
Recently, an austenite stainless steel of 16Cr--14Ni--4Si system containing about 4% Si and forming a surface oxide layer mainly constituted by SiO.sub.2 is finding a wide use as a metallic material which exhibits a large resistance to oxidation under the above mentioned strong oxidizing environment. Unfortunately, however, even this austenite stainless steel cannot provide satisfactory corrosion resistance in a hot nitric acid environment having a high concentration exceeding 98%.
Under these circumstances, the present inventors have developed and proposed in Japanese Patent Publication No. 9626/1982 (Japanese Patent Laid-Open No. 91960/1980) a high Si austenite stainless steel having a composition containing not greater than 0.03% of C, 5 to 7% of Si, not greater than 10% of Mn, 7 to 16% of Cr, 10 to less than 19% of Ni and, as required, 4.times.[C] to 2% of at least one of Ti, Ta, Zr and Nb, and the balance Fe and inevitable impurities. This stainless steel exhibits excellent workability and weldability, as well as a high resistance to oxidation in the presence of nitric acid of high concentration and, hence, can be used satisfactorily even in such a strongly corroding environment.
When such an austenite stainless steel having high Si content is subjected to welding, a large quantity of intermetallic compounds of Ni--Si--(Nb, Zr) system are generated in the weld metal. The weld metal containing such intermetallic compounds exhibits an impractically low impact strength at ordinary temperature on the order of 0.5 Kg-m/cm.sup.2 under the as welded condition, so that the austenite stainless steel mentioned above cannot be used suitably as the structural material. Therefore, a welding material of 16Cr--14Ni--4Si system is used in the case where the weld part is required to have a high impact toughness. Such a welding material, however, undesirably reduces the corrosion resistance of the weld part due to the insufficient Si content. In general, MIG or TIG welding method is adopted in the welding of the metallic materials of the type described above, so that the composition of the welding material can be regarded as being materially identical to the composition of the weld metal.